Non-equilibrium molecular dynamics simulations of structured molecules

The failure of extended corresponding-states techniques to predict the viscosity of low molecular weight branched and cyclic hydrocarbons suggests that molecular structure greatly affects the viscosities of these species. To investigate the inter-relationship of structure and viscosity, non-equilibrium molecular dynamics simulations of Lennard-Jones site-site models representing selected simple molecules were performed. The simulation conditions nearly span the liquid density range over which experimental data are currently available. In this study, models representing n-hexane, cyclohexane and benzene were constructed using six equivalent sites, each characterized by the same Lennard-Jones parameters used in a previous study of n-butane and isobutane viscosities. The focus was, therefore, entirely on structural effects. Simulated viscosities agreed well with experimental values of n-hexane and cyclohexane indicating that structural differences primarily account for the differences in viscosity between these two fluids. Simulated benzene viscosities were substantially higher than experimental values, indicating important non-structural effects. The relationship between fluid structure and the applied shear field are discussed.